This invention relates to titanium on silica catalysts and methods for preparing such catalysts.
Titanium on silica catalysts have been known to be effective in catalyzing epoxidation reactions. For example, U.S. Pat. No. 4,021,454 to Wulff et al. describes the use of such catalysts to epoxidize substituted olefins, such as allyl methyl ether to form 2,3-epoxypropyl methyl ether. Titanium on silica catalysts can also be used in other reactions including, but not limited to, olefin polymerization, hydroxylation, and isomerization.
A number of characteristics are important in determining the usefulness of titanium on silica catalysts. In addition to the physical strength and the attrition resistance of a catalyst, the activity of a catalyst, defined by the reaction rate per unit weight of catalyst, is an important characteristic. In general, it is believed that the activity of a titanium on silica catalyst is dependent on the amount of active titanium present on the silica gel. As used herein, the phrase xe2x80x9cactive titaniumxe2x80x9d means a titanium compound which is chemically bound to the silica gel and serves to facilitate whatever reaction (e.g., epoxidation) which that catalyst is used for. Such active titanium typically exists in the form of Ti(OH)x, wherein x is 1, 2, or 3, with titanium also typically bound to 1, 2, or 3 silicon atom(s) inherent to the silica gel.
Not only is the amount of titanium on the silica gel important in most reactions, the distribution of titanium on a given formed silica gel particle, for example, a macrosphere, an extrudate, or a pellet, is important in many applications. Formed silica gel particles such as macrospheres, extrudates, or pellets are known to those skilled in the art. The term xe2x80x9cmacrospherexe2x80x9d is discussed in more detail below but, in general, is a conglomeration of silica gel particles formed into a spherical shape upon ejection of a silica hydrosol solution from a nozzle. The kinetics of the particular reaction for which the catalyst is used are relevant for determining the type of distribution of titanium on a given formed silica gel particle. For example, if the reaction must occur very quickly (e.g., the system provides for only a short residence time of the reactants with the catalyst), it would be optimal to use a macrosphere having an eggshell type distribution. As used herein, the phrase xe2x80x9ceggshell distributionxe2x80x9d means titanium concentration levels along an exposed diameter of a macrosphere which generally peak near the ends of the diameter and flatten towards the middle and specifically at least 60% of the total titanium is within 20% of both ends and the minimum concentration towards the middle is no more than 10% of the peak concentration. If a macrosphere having a uniform distribution is used for such a reaction, then some of the titanium in the interior of the macrosphere would not be utilized. Such reactions include the combustion of fuel in an automotive vehicle.
On the other hand, a uniform distribution of titanium is desirable in some systems. As used herein, the phrase xe2x80x9cuniform distributionxe2x80x9d means titanium concentration levels along an exposed diameter of a macrosphere which do not vary by more than 20% from an average titanium concentration at any one point along the diameter, excluding data points within 5% of each end of the diameter at which points data from known analytical methods can be unreliable. Characteristics of such systems might include reactants which react relatively slowly in relation to the time it takes for the reactants to diffuse into the macrospheres. In such cases, an eggshell distribution would be inefficient in that some of the reactants which had diffused to the interior of the macrosphere would be lacking active titanium sites. Such reactions might possibly include hydroxylation. It might be desirable to prepare a silica gel macrosphere having a titanium distribution between uniform and eggshell. As used herein, the phrase xe2x80x9cintermediate distributionxe2x80x9d shall include all types of titanium distributions other than uniform and eggshell.
To date, there is no indication of how to control the type of distribution of titanium on a silica gel macrosphere. For the reasons discussed above, such a method would be useful.
The present invention is directed to methods for a catalyst having a particular titanium distribution type, which can be controlled by selecting the particular titanium precursor and the molar titanium:hydroxyl ratio. By varying one or both of these factors, either a uniform, eggshell, or intermediate distribution of titanium can be achieved or more closely approximated.
The method of the present invention is carried out by first preparing formed silica hydrogel particles containing water and having a hydroxyl concentration and a hydroxyl distribution, then drying the formed silica hydrogel particles to remove substantially all of the water. After selecting a particular titanium precursor having a certain reactivity with hydroxyl groups, the formed silica gel particles are contacted with the titanium precursor in an amount to achieve a molar titanium:hydroxyl ratio. It has been recognized that the primary factors in determining the titanium distribution type are the reactivity of the titanium precursor and the molar titanium:hydroxyl ratio. Specifically, increasing the reactivity and/or decreasing the molar titanium:hydroxyl ratio aids in forming an eggshell distribution, while decreasing the reactivity and/or increasing the molar titanium:hydroxyl ratio aids in forming a uniform distribution. By contacting the formed silica gel particles with the titanium precursor, titanium-impregnated formed silica gel particles are formed then recovered as the catalyst.
The present invention also provides a method for preparing a catalyst having either a uniform distribution or an eggshell distribution, or more closely approximating one of these distributions. In order to attain or more closely approximate a catalyst having a uniform distribution, a titanium precursor with a relatively low reactivity is used and/or a relatively high molar titanium:hydroxyl ratio is used. On the other hand, in order to attain or more closely approximate a catalyst having an eggshell distribution, a titanium precursor with a relatively high reactivity is used and/or a relatively low molar titanium:hydroxyl ratio is used.
The present invention also provides methods for preparing a catalyst having a uniform distribution of titanium and for preparing a catalyst having an eggshell distribution of titanium by using specific titanium precursors and specific molar titanium:hydroxyl ratios. In particular, to prepare a catalyst having a uniform distribution of titanium, the titanium precursor is selected from the group consisting of titanocene, titanium acetylacetonate, isopropyl titanate-acetylacetone complex, and triethanolamine titanate and the titanium precursor is added in an amount to achieve a molar titanium:hydroxyl ratio from about 0.25:1 to about 3:1. Similarly, to prepare a catalyst having an eggshell distribution of titanium, the titanium precursor is selected from the group consisting of ethyl titanate, n-propyl titanate, isopropyl titanate, isobutyl titanate, and n-butyl titanate and the titanium precursor is added in an amount to achieve a molar titanium:hydroxyl ratio from about 0.03:1 to about 0.25:1.
The invention is also directed to the titanium on silica catalysts made by the processes described herein.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.